1. Field
The invention relates generally to wireless communications, and more specifically to data transmission in a multiple access wireless communication system.
2. Background
An access channel is used on the reverse link by an access terminal for initial contact with an access point. The access terminal may initiate an access attempt in order to request dedicated channels, to register, or to perform a handoff, etc. Before initiating an access attempt, the access terminal receives information from the downlink channel in order to determine the strongest signal strength from nearby access points and acquire downlink timing. The access terminal is then able to decode the information transmitted by the given access point on a broadcast channel regarding choice of parameters governing the access terminal's access attempt.
In some wireless communication systems, an access channel refers both to a probe and message being rendered. In other wireless communication systems, the access channel refers to the probe only. Once the probe is acknowledged, a message governing the access terminal's access attempt is transmitted.
In an orthogonal frequency division multiple access (OFDMA) system, an access terminal typically separates the access transmission to be transmitted on the access channel into parts, a preamble transmission and a payload transmission. To prevent intra-cell interference due to lack of fine timing on the reverse link during the access preamble transmission, a CDM-based preamble transmission may be time-division-multiplexed with the rest of the transmissions (i.e., traffic, control, and access payload). To access the system, the access terminal then randomly selects one PN sequence out of a group of PN sequences and sends it as its preamble during the access slot.
The access point searches for any preambles (i.e., all possible PN sequences) that may have been transmitted during the access slot. Access preamble transmission performance is measured in terms of collision probability, misdetection probability and false alarm probability. Collision probability refers to the probability that a particular pseudo-random (PN) sequence is chosen by more than one access terminal as its preamble in the same access slot. This probability is inversely proportional to the number of preamble sequences available. Misdetection probability refers to the probability that a transmitted PN sequence is not detected by the base station. False alarm probability refers to the probability that an access point erroneously declared that a preamble has been transmitted while no preamble is actually transmitted. This probability increases with the number of preambles available.
The access point then transmits an acknowledgment for each of the preambles detected. The acknowledgement message may include a PN sequence detected, timing offset correction, and index of the channel for access payload transmission. Access terminals whose PN sequence is acknowledged can then transmit the respective access payload using the assigned resource.
Because the access point has no prior knowledge of where the access terminal is in the system (i.e. what its power requirements, buffer level, or quality of service may be), the acknowledgement message is broadcasted at a power level high enough such that all access terminals in the given cell can decode the message. The broadcast acknowledgement is inefficient as it requires a disproportionate amount of transmit power and/or frequency bandwidth to close the link. Thus, there is a need to more efficiently send an acknowledgment message to access terminals in a given cell.